1. Field of the Invention
The invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device that includes a driving circuit composed of polycrystalline silicon thin film transistors.
2. Discussion of the Related Art
Liquid crystal display (LCD) devices are developing as the next generation of display devices because they are portable and consume little power. In general, an LCD device includes two substrates disposed such that respective electrodes of the two substrates face into each other. A liquid crystal layer is interposed between the respective electrodes. When a voltage is applied to the electrodes, an electric field is generated. The electric field modulates the light transmittance of the liquid crystal layer by reorienting the liquid crystal molecules, thereby displaying images in the LCD device.
One substrate of an LCD device includes a thin film transistor (TFT) that acts as a switching device. The TFT is often formed using amorphous silicon as an active layer. One reason for this is that amorphous silicon can be formed on a large, low cost substrate such as glass. LCD devices also include drive integrated circuits (drive ICs) that control the TFT. Unfortunately, amorphous silicon does not form a suitable active layer for drive ICs, which are usually CMOS (complementary metal-oxide-semiconductor) devices that require an active layer of single crystalline silicon. Because of this, drive ICs are usually connected to a TFT substrate using a TAB (tape automated bonding) system. This adds significant cost to LCD devices.
Because of the limitations of amorphous silicon, LCD devices that incorporate a polycrystalline silicon TFT, in which polycrystalline silicon is used as an active layer, are under research and development. Polycrystalline silicon is highly preferred because it is better suited for use in a drive IC than amorphous silicon. Polycrystalline silicon thus has the advantage that the number of fabrication steps can be reduced because thin film transistors and drive ICs can be formed on the same substrate, thus eliminating the need for TAB bonding. Furthermore, the field effect mobility of polycrystalline silicon is 100 to 200 times greater than that of amorphous silicon. Polycrystalline silicon is also optically and thermally stable.
Among many recent methods of forming polycrystalline silicon, a new method of crystallization, often referred to as sequential lateral solidification (SLS), has become of interest. The SLS method takes advantage of the fact that silicon grains grow laterally from the phase boundary between liquid silicon and solid silicon. The SLS method can increase the size of the silicon grains by controlling the energy intensity of a laser beam and the irradiation range of the laser beam used to grow the silicon grains.
FIG. 1 shows a schematic view of an apparatus for the related art SLS method.
FIG. 1 depicts an apparatus for the SLS method that includes a light source 1, an attenuator 3, a focusing lens 9, a mask 11, an imaging lens 13, and a moving stage 19 on which a sample 17 having an amorphous silicon layer 20 (of FIG. 2A) is situated. The apparatus for the SLS method also includes first to third reflective mirrors 5, 7, and 15 to change the direction of the light. The first and second reflective mirrors 5 and 7 are disposed between the attenuator 3 and the focusing lens 9, and the third reflective mirror 17 is disposed between the imaging lens 13 and the moving stage 19.
The light source 1 is preferably a XeCl (xenon-chloride) excimer laser having a wavelength of 308 nm. The attenuator 3 controls the energy of the laser beam through the system. The focusing lens 9 and the imaging lens 13 condense the laser beam, while the focusing lens 9 makes the intensity of the laser beam more uniform by equalizing focus lengths of the laser beam. The mask 11 forms the laser beam into a predetermined shape.
The laser beam from the light source 1 therefore transmits through the attenuator 3 and is reflected by the first and second reflective mirrors 5 and 7. The laser beam is then condensed by the focusing lens 9, shaped by the mask 11, and passes through the imaging lens 13. The laser beam is next reflected by the third reflective mirror 15 onto the sample 17. The moving stage 19 then moves the sample 17, and irradiation is repeated.
FIGS. 2A to 2C show schematic plane views depicting a process for crystallizing an amorphous silicon film using the related art apparatus for the SLS.
In FIG. 2A, a first laser beam irradiation has been carried out at a first region “A” of an amorphous silicon film 20. As discussed above, the silicon grains grow laterally from the boundary between liquid phase silicon and solid phase silicon to result in first grains 22 growing from both sides of the first region “A.” Grain growth stops at a first line “IIa” where the first grains 22 meet.
FIG. 2B shows the results of when a second laser beam irradiation is carried out at a second region “B” of the amorphous silicon film 20. The second region “B” includes part of the first region “A.” In a first overlapping region “AB,” where the first region “A” and the second region “B” overlap, the grains 22 (of FIG. 2A) act as crystallization seeds. Grain growth stops at a second line “IIb” where second grains 23 meet. The second grains 23 are larger than the first grains 22, which were formed after the first laser beam irradiation.
FIG. 2C shows a third laser beam irradiation accomplished at a third region “C” of the amorphous silicon film 20. Third grains 24 grow from boundaries of the third region “C”. The third region “C” includes part of the second region “B.” In the second overlapping region “BC,” where the second region “B” and the third “C” region overlap, the second grains 23 (of FIG. 2B) act as crystallization seeds. The third grains 24 are therefore much larger than the second grains 23 (of FIG. 2B).
Repeated laser beam irradiation scans the whole amorphous silicon film 20 to create polycrystalline silicon with large grains. Furthermore, high crystallization productivity results from the small number of times the same point is irradiated.
However, a polycrystalline silicon film formed by the SLS method tends to have different-sized grains and irregular growing directions. Thus, TFTs fabricated from the polycrystalline silicon film by the SLS method also have properties that depend on the grain-growth direction and grain boundary.
FIG. 3 shows a schematic graph showing a property of a thin film transistor using a polycrystalline silicon film grown by the related SLS method.
In FIG. 3, the x-axis indicates the TFT gate voltage (Vg) and the y-axis indicates the TFT drain current (Id). Each line also shows a first case (solid lines) in which the directions of the channel-passed current and the grain-growth are parallel, a second case (short dotted lines) in which the directions of the channel passed current and the grain-growth direction form an angle of 45 degrees, and a third case (long dotted lines) in which the directions of the channel passed current and the grain-growth have an angle of 90 degrees. These lines are established at drain voltages “Vd” of 0.1 V and 10 V. FIG. 3 shows that reducing the angle between the channel direction and the grain-growth direction decreases the number of the grain boundaries. This improves the current-voltage characteristics. Therefore, if the channel direction of a TFT runs parallel with the grain-growth direction, then the TFT properties are enhanced. However, if the channel direction of a TFT and the grain-growth direction are at an angle of 90 degrees, then the properties of TFT are minimized.
FIGS. 4 and 5 show schematic plane views depicting thin film transistors formed using a polycrystalline silicon film made by the SLS method according to first and second related art embodiments, respectively. FIGS. 4 and 5 have TFTs with a grain size of the polycrystalline silicon film smaller than a channel area “ch” of the TFT.
The current path of the channel area “ch” and the grain-growth direction make an angle of 90 degrees In FIG. 4, while the current path is parallel to the grain-growth direction in FIG. 5. Since the TFT of FIG. 5 is less influenced by the grain boundary than that of FIG. 4, the TFT of FIG. 5 has superior characteristics to that of FIG. 4. A TFT using a polycrystalline silicon film by the SLS method accordingly has optimal characteristics when the current path through the channel coincides with the grain-growth direction, and the TFT has the most inferior characteristics when the current path through a channel is perpendicular to the grain-growth direction. Thus, when a polycrystalline silicon film made by the related art SLS method is used for a TFT, a uniform property of the TFT can not be obtained. Moreover, even when the best TFT is obtained, the characteristics of the TFT are not sufficient for a drive IC, but merely for a gate driver and a data driver.